Chironomid communities of the upper St. Croix River, Wisconsin, were sampled for pupal exuviae at four locations monthly from April to October, 2007. Species richness was very high, with 252 species from 73 genera, dominated by the subfamilies Chironominae and Orthocladiinae. Most studies of lotic systems find fewer than 150 chironomid taxa, and often less than 100. The high richness may be due to regional conditions that support diverse aquatic communities, such as thermal regime, typical landscape patterns such as elevated mid-order stream diversity or increased β-diversity in headwaters, or the collection method which can detect species from adjacent habitats. There were 35 species that are atypical of lotic systems and some may have occupied microhabitats or adjacent habitat that more closely matches their preferences. Twenty-one species, mostly in Orthocladiinae, had range expansions into the western Great Lakes region or appear to be previously undescribed for the Nearctic. Sixty morphotypes, dominated by 41 Chironominae, did not fit any published exuviae descriptions.